Electronic devices use power to operate. Switched mode power supplies are commonly used due to their high efficiency, small size and low weight to power many of today's electronics. Conventional wall sockets provide a high voltage alternating current. In a switching power supply a high voltage alternating current (ac) input is converted to provide a well regulated direct current (dc) output through an energy transfer element. In operation, a switch is utilized to provide the desired output by varying the duty cycle (typically the ratio of the on time of the switch to the total switching period), varying the switching frequency or varying the number of pulses per unit time of the switch in a switched mode power supply.
In one type of dimming for lighting applications, a triac dimmer circuit typically removes a portion of the ac input voltage to limit the amount of voltage and current supplied to an incandescent lamp. This is known as phase dimming because it is often convenient to designate the position of the missing voltage in terms of a fraction of the period of the ac input voltage measured in degrees. In general, the ac input voltage is a sinusoidal waveform and the period of the ac input voltage is referred to as a full line cycle. As such, half the period of the ac input voltage is referred to as a half line cycle. An entire period has 360 degrees, and a half line cycle has 180 degrees. Typically, the phase angle is a measure of how many degrees (from a reference of zero degrees) of each half line cycle the dimmer circuit removes. As such, removal of half the ac input voltage in a half line cycle by the triac dimmer circuit corresponds to a phase angle of 90 degrees. In another example, removal of a quarter of the ac input voltage in a half line cycle may correspond to a phase angle of 45 degrees.
Although phase angle dimming works well with incandescent lamps that receive the altered ac input voltage directly, it typically creates problems for light emitting diode (LED) lamps. LED lamps require a regulated power supply to provide regulated current and voltage from the ac power line. Conventional regulated power supply controllers typically don't respond desirably to a removal of a portion of the ac input voltage by a triac dimmer circuit. Conventional power supplies are typically designed to ignore distortions of the ac input voltage and to deliver a constant regulated output until a low input voltage causes them to shut off completely. Thus, as a result of phase dimming, a conventional power supply is likely to produce unacceptable results such as flickering of the LED lamp, flashing of the LED lamp at high phase angles, and color shifting of the LED lamp.
Thus, a conventional power supply controller may be designed to implement dimming of the output of the power supply in response phase dimming at the input, such as by a triac dimmer. One type of dimming control used by conventional power supply controllers is referred to as closed loop dimming. For closed loop dimming control, the conventional power supply controller may actively sense one or more output quantities to regulate the output of the power supply. In addition, the conventional power supply controller that is designed for phase dimming may respond to triac dimmer by directly sensing the dimmer voltage output (in other words, the ac input voltage after the triac dimmer circuit has removed a portion of the ac input voltage) to determine the amount of dimming requested. However, at high phase angles (corresponding to the triac dimmer circuit removing a large portion of the ac input voltage) the triac dimmer output becomes less symmetric. In other words, even though the triac dimmer is set to provide a constant phase angle, variances remain in the resultant phase angle at every half line cycle resulting in the triac dimmer output being not symmetric. The asymmetry of the triac at high phase angles tends to produce unacceptable results such as flickering of the LED lamp. Closed loop dimming control generally magnifies unacceptable results such as flickering since the power supply controller would be responding to a changing output power of the power supply (due to the changing phase angle) at every half line cycle.
Another type of dimming control used by conventional power supply controllers does not sense the dimmer voltage output but instead utilizes the inability of the power supply to regulate at low input voltages. As mentioned above, the power supply provides a constant regulated output until a low input voltage causes them to shut off completely. However, the shut off of the power supply is not instantaneous. Instead, the power supply becomes unable to regulate the output at a low input voltage and causes the output to begin dimming. The triac dimmer circuit removes enough of the ac input voltage such that it causes the output of the power supply to begin dimming. As such, dimming often occurs at high phase angles. When dimming occurs at high phase angles, the dimming transition occurs quickly since the range of dimming is small. In addition, depending on the load of the power supply, dimming may occur at different phase angles.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.